sir i have given 12th board exam in 2017-18 and reappear in 2018-19. sir am i eligible for jee advance?

Hello there,
Hope you are well.
Jee advanced 2019 has already been conducted,  and you was eligible for appearing in the exam. A student has a maximum of 3 chances to attempt JEE mains and hence jee advanced, the year you passed 12th doesn't matter,  you was eligible for JEE Advanced 2019.

Hello aspirant!
Answering your question-
JEE Main is applicable for admission to NITs, IIITs and CFTIs participating though Central Seat Allocation Board subject to the condition that the candidate should have secured at least 75% marks in the 12th class examination, or be in the top 20 percentile in the 12th class examination conducted by the respective Boards. For SC/ST candidates the qualifying marks would be 65% in the 12th class examination.

If the candidate appeared for improvement exam-
If a candidate passes Class 12 but writes one or a few subjects for improvement or any other reason, then top 20 percentile cut-off will be considered.

However, if a candidate writes all the subjects then the best of the two performances will be considered.

In case a Board does not provide information about the cut-off for the top 20 percentile, the candidate will have to produce a certificate from the respective Board stating that he/she falls within the top 20 percentile of successful candidates. If the candidate fails to do so, then the cut-off marks for the CBSE will be used.

If a candidate has passed Class 12th/ Equivalent qualifying examination from one state but appeared for improvement from another state, their state code of eligibility will be from where they have originally passed Class 12th/ Equivalent qualifying examination and not the state from where they have appeared for improvement.
Hope this helps you.